The present invention relates generally to a thermal printing device for use in facsimiles or similar image reproduction equipment, and particularly to a thermal printing head for printing multiple color or graduated tone images on paper coated with thermo sensitive chemical.
In the commonly available thermal printing devices, a thermal printing head is provided with multiple rows of heating spots over its surface extending in the direction of the scanning lines as indicated by arrow Q in FIG. 1. Each of these rows divided from a single bar of heating resistor 1 is substantially uniform in thickness and width along the length. As shown in FIG. 1 which illustrates a heating resistor of the thermal printing head of a conventional thermal printing device, equal numbers of electrodes "a" and "b" (each suffixed with a sequential number in the drawing, for example, a-1, a-2, a-3 and so on) are laid laterally in an alternative manner on the heating resistor 1. From the electrodes "a" and "b", electric power is selectively supplied. By each pair of the adjacent electrodes "a", "b", the heating resistor 1 is divided into heating surfaces or heating spots, "h" (represented in the drawing as h-1, h-2, h-3 and so on) which produces heat energy when the associated electrode pair are energized. By appropriately selecting the electrode pairs to be energized, a desired pattern of the heating surfaces to heat up the thermo sensitive paper is formed. As a result, the thermo sensitive chemical on the paper will be thermally stimulated to develop an image exactly outlined by the pattern. These prior art devices have been very effective only where monochrome reproduction is required.
Currently, however, progressively increasing emphasis has been placed on the need for multiple color tone graduation reproduction. For multiple color or tone development, different types of thermo sensitive paper are used which produce varying colors when exposed to different levels of temperature. If any of these conventional devices is to be employed for this purpose, it has to be equipped with an extremely complicated control circuit capable of varying the current in voltage or time span of conduction to each programmed heating surface.